Diesel emission standards for vehicle engines are becoming increasingly stringent. It has been impossible to meet emissions legislations by merely relying on the diesel engine itself.
Thus, various exhaust aftertreatment devices have played an essential role in engine emission technologies. Diesel particulate filters (DPF) are in common use for particulate matter (PM) control, and lean NOx traps (LNT) are used for NOx control in light-duty diesel applications. In addition to diesel applications, LNT's can be used with lean burn gasoline engines.
Under lean conditions, an LNT adsorbs oxides of nitrogen (NOx) produced from engine combustion. The adsorption process generally involves two steps. First, engine-out nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2) on an active oxidation catalyst (such as platinum). Second, the NO2 is adsorbed in the form of nitrates by a storage material (such as barium oxide). The LNT may be regenerated under rich conditions, whereby NOx is released and reduced to N2 over a reduction catalyst (such as rhodium).
Sulfur poisoning is a particularly challenging problem for LNT devices. Lean-burn engine exhaust contains oxides of sulfur (SOx), derived from fuel and lubricating oil, which compete with NOx for LNT adsorption sites. Unfortunately, SOx is preferentially adsorbed over NOx and forms stable sulfates with the LNT storage materials. As a result, LNT performance gradually declines because fewer storage sites are available for NOx adsorption.
Even if ultra low sulfur fuel is used, without effective sulfur management, an LNT can be fully de-activated over time. To effectively manage sulfur poisoning, high temperature desulfation is periodically performed. Desulfation requires a high temperature exhaust (e.g. 650° C.) and rich conditions to release sulfur from the LNT adsorption sites.